This invention relates to a technique for reducing degradation and deterioration of video signals which, after being reproduced from a digital record medium are subjected to analog processing and, more particularly, to such a technique which uses a multi-loop test for anticipating and correcting such deterioration and degradation.
Digital video recorders are known to provide video signals of high quality which, when subjected to digital processing, suffer little, if any, deterioration. Such digital video signals may be reproduced, recorded and re-recorded several times; and the inherent characteristics of digital signals and digital processing permit repeatability of the reproduction of such digital video signals without loss of quality. However, when digital video signals are subjected to analog processing, such as conversion from digital to analog form, analog recording, analog transmission, and the like, the resultant analog signals may be degraded in comparison to the original digital video signals. If the analog video signal is dubbed and re-dubbed several times, such degradation is multiplied and deterioration of the video signal and video picture ultimately reproduced therefrom becomes quite noticeable. Even if the analog video signal is converted to digital form, as by an analog-to-digital converter, for digital processing and recording, the inherent quantizing noise associated with such A/D conversion adds to the deterioration of the video signal.
Typically, degradation of the video signal appears as errors in the amplitude, frequency, phase or reference signal levels (e.g. the video black signal level) in the analog video signal. Such errors often are not uniform and constant but, rather, vary as a function of the analog processing to which the video signal is subjected, as well as the apparatus and operating characteristics used to process, record, reproduce or transmit the video signals. If apparatus which tends to introduce the aforenoted errors are connected in cascade, the deterioration of the video signal caused thereby is aggravated.
In anticipation of such degradation of the video signals caused by analog recording and reproduction thereof, recording/reproducing apparatus often is adjusted, or pre-set during manufacturing to compensate or pre-correct for expected errors. However, such pre-settings often must be readjusted by the end user of the apparatus, which is a difficult, time-consuming and laborious task. Indeed, manual adjustments to minimize degradation of the video signal that is dubbed or re-dubbed often cannot be determined until several successive dubbing operations have been carried out. Furthermore, even when video signal degradation is recognized sufficiently to permit correction, it has been found that when an error associated with one type of characteristic in the video signal is corrected, errors appear in other characteristics. Thus, pre-correction adjustments often are dependent upon each other, which require several trial and error adjustment iterations of individual video signal characteristics until the user is satisfied. This difficulty is compounded when it is recognized that the errors for which such pre-correction adjustments are made often are so small that they can be detected only with difficulty. For example, when a digital video signal is reproduced, converted to analog form, recorded by an analog recorder, subsequently reproduced and displaced as an analog video signal, errors in the displayed video signal relative to the original digital video signal may be so slight as to be practically imperceptible. However, if that analog video signal subsequently is recorded and reproduced several times, its degradation relative to the original digital video signal would be clearly pronounced and noticeable.
It has been proposed heretofore to make successive recordings of a video signal and thereby subject that signal to successive degradations in order to detect the loss of quality attributed to such recordings. For example, the video signal is reproduced, delayed, fed back to be re-recorded and then reproduced once again; and this cycle is repeated several times to permit observation of deteriorations occasioned by such re-recordings. Japanese Patent Publication 50-3111 (1975) broadly describes this technique.
A similar arrangement is described in International Patent Application WO 87/06420 (1987) in which a video tape recorder is included in a loop to record in successive tracks a single field or frame of video signals that has been cycled through the loop. An initial field or frame, supplied from an external source, is recorded and practically simultaneously reproduced. This reproduced field or frame is converted to digital form in a timebase corrector and then re-converted to analog form for re-recording in the next track by the VTR. After a number of cycles, or generations, deterioration in the field or frame recorded in the nth track may be observed, thus permitting a user to effect manual adjustments to the VTR, the timebase corrector and/or the ADC and DAC. However, with this system, it is quite difficult to accurately determine the degradation to which the original field or frame has been subjected. To do so requires that the initial field or frame be reproduced and displayed and then the nth generation of that field or frame must be reproduced and displayed. This is quite time-consuming and requires the precise, rapid location of and access to the first and nth tracks on the video tape. Moreover, manual adjustments often must be made on an iterative basis, and this is not satisfactory.